What happened
On 4 June 1998, a Jetstream 4100, registration G-MAJE, was conducting a scheduled passenger flight from Belfast to Southampton. While cruising at 19,000 feet, the crew observed a 'TRIM L' warning on the cockpit panel. Although the aileron trim indicator appeared neutral, the First Officer found that the aileron trim control had become completely stuck.
Upon disengaging the autopilot, the crew experienced significant difficulty maintaining level flight, requiring substantial physical force on the control wheel to prevent the aircraft from rolling to the right. The flight crew declared a PAN call and requested radar vectors from Air Traffic Control to divert back to Belfast. As the aircraft descended through lower altitudes, the control stiffness began to diminish. By the time the aircraft reached 14,000 feet, the handling and trim controls had returned to normal operation, allowing for a safe landing at Belfast with no injuries and no damage to the aircraft.
The investigation
Investigators examined the aircraft's aileron trim system, which utilizes a mechanical screwjack actuator to adjust the balance tab on the left aileron. The investigation focused on the lubrication used in the actuator, noting that the manufacturer had recently updated the maintenance manual to specify Mobil 28 grease, which is less hygroscopic than the previously used Aeroshell Grease 7.
Analysis of the flight data recorder confirmed that the aircraft experienced two roll excursions to the right shortly before the autopilot was disengaged. The investigation also reviewed previous maintenance records, which showed that moisture contamination had been found in the actuator during a similar incident on the same aircraft just weeks prior. Subsequent inspections of the screwjack and cable/pulated system following the June incident revealed no mechanical faults, though the operator implemented new lubrication procedures to ensure any excess grease was cleaned from the ram.
Findings
- The primary cause of the control difficulty was the freezing of water within the aileron trim actuator.
- It is believed that moisture entered the unsealed actuator through clearances in the bearing body or splined body.
- The presence of moisture was likely due to the original grease not being fully purged during previous maintenance or cleaning operations.
- A lateral trim imbalance developed during cruise, which may have been influenced by changes in airspeed.